The present invention relates to electrochromic devices exhibiting a reversible change in color or transparency upon the application of an electric field. Specifically, the invention relates to electrochromic devices incorporating a novel electrolyte layer consisting of a porous glass sheet containing a salt or other ion-conductive compound in the pore structure thereof.
U.S. Pat. No. 3,521,941 discloses an electrochromic device of the type wherein a thin layer of a selected inorganic oxide or other compound provides the electric-field responsive electrochromic characteristic. Included in the device is an adjoining layer, called a current-carrier-permeable insulator, which constitutes the electrolyte or source of mobile ions for the operation of the device. Plastics as well as metal oxides, sulfides, fluorides, and nitrides are mentioned as suitable electrolyte materials.
It has been proposed to use colorless solid ionic conductors as electrolytes in electrochromic devices. Hence, U.S. Pat. No. 3,712,710 suggests the use of aluminum oxide combined with an alkali or alkaline earth metal oxide such as Na.sub.2 O, K.sub.2 O or MgO to provide the electrolyte film.
Faster response in electrochromic devices containing films of WO.sub.3 and/or MoO.sub.3 as the electrochromic layer has been obtained by using liquid or gel electrolytes, typically based on sulfuric acid. U.S. Pat. No. 3,708,220 suggests a gelled acid electrolyte of this type, while U.S. Pat. No. 4,021,100 discloses the use of a porous insulating layer of alumina, silica or glass saturated with a liquid acid electrolyte. U.S. Pat. No. 4,175,837 proposes the use of liquid electrolytes containing selected sodium or lithium salts.
Although liquid and gel electrolytes such as described in the foregoing patents impart good electrochromic performance, problems relating to the handling and containment of the liquid or gel remain. In addition, the preferred WO.sub.3 electrochromic materials are attacked by acidic electrolyte materials, limiting the utility of strong acids for this application.
A newer proposed class of solid electrolytes, disclosed in U.S. Pat. No. 3,995,943, includes certain electrically insulating silver compounds permeable to silver ions. An example of such an electrolyte is Ag.sub.4 RbI.sub.5. U.S. Pat. No. 4,106,862 suggests the use of ionic conductors of the formula Na.sub.1-x Zr.sub.2 Si.sub.x P.sub.3-x O.sub.12, wherein x ranges from 0.8 to 2.4, as electrically insulating, sodium ion-conducting electrolyte materials. However, these electrolytes are relatively expensive to prepare.
It is therefore a principal object of the present invention to provide an electrochromic device incorporating a new solid electrolyte material offering good electrochromic response when used with known electrochromic materials.
It is a further object of the invention to provide an electrochromic device comprising a porous glass sheet as the electrolyte matrix.
Other objects and advantages of the invention will become apparent from the following description thereof.